1. Field of the Invention
The present invention relates to a semiconductor memory device having a ferroelectric capacitor and a manufacturing method thereof.
2. Description of the Related Art
In recent years, wide investigations have been being conducted to develop the non-volatile semiconductor memory device with the capacitor in which polarization characteristics of the ferroelectric material are made good use of.
The ferroelectric materials used in these non-volatile semiconductor memory devices are crystals with the perovskite structure, and, among these materials, lead zirconate titanate (PZT), expressed by Pb(ZrxTi1-x)O3, is generally regarded to be typical one.
As a capacitor utilizing a ferroelectric material, there has been disclosed, for example, in Japanese Patent Application Laid-open No. 31399/2000 a dielectric element with a structure in which two electrodes are separated by a dielectric substance of Pb(ZrxTi1-x)O3, wherein the Zr composition ratio x is smaller in the vicinity of the afore-mentioned electrodes than in the central section of the afore-mentioned dielectric substance. Further, it is therein described that, by setting the composition in the vicinity of the electrodes fit to excel in lattice matching and the composition in the central section of the dielectric substance fit to excel in polarization characteristics, this structure can raise the remanent polarization and provide the dielectric substance free from a risk of the film peeling-off.
Further, in Japanese Patent Application Laid-open No. 67650/2000, there is disclosed a ferroelectric thin film element which comprises a conductive thin film formed on a single crystal substrate and a thin film of a Pb(Zr, Ti)O3-based ferroelectric substance with the perovskite structure formed on the afore-mentioned conductive thin film, the afore-mentioned ferroelectric thin film being formed of a first layer having the Zr content which gradually increases with the film thickness from the interface with the afore-mentioned conductive thin film and a second layer formed to overlie the afore-mentioned first layer and have a constant Zr content throughout, wherein the compositions of the first layer and the second layer in the vicinity of the border between these two layers are practically identical. Further, it is described therein that this structure can provide a ferroelectric thin film element having a ferroelectric thin film of good orientation with a few crystal defects.
Meanwhile, in Japanese Patent Application Laid-open No. 58525/2000, there is disclosed a method of forming a ferroelectric film such as PZT by the metal organic chemical vapor deposition (MOCVD) method, wherein the film deposition can be carried out at a low temperature and a ferroelectric film of good orientation and excellent crystalline quality can be obtained without degrading plugs, interconnections or transistors in lower layers. More specifically, on a conductive material, the early-nucleation in the perovskite crystal structure is made under first conditions of the film deposition, and then, on the early-nucleus, a film with the perovskite crystal structure is grown under second conditions of the film deposition. Thereat, the early-nucleation under first conditions of the film deposition is made either under the condition that the feeding rate of Zr is reduced in comparison with that under second conditions of the film deposition or under the condition the source gas of Zr is not fed at all.
Further, in Japanese Patent Application No. 336083/2001. it is mentioned that, using the method of the film deposition described in Japanese Patent Application Laid-open No. 58525/2000, the Zr/Ti ratio in the ferroelectric film is made to increase from the side of a lower electrode towards the side of an upper electrode, with the object of reducing the lattice distortion on the interface between the early-nucleus lying on the conductive material and the ferroelectric film formed thereon. That is, by making the Zr/Ti ratio on the side of the lower electrode small, the lattice distortion may be reduced, and by making the Zr/Ti ratio towards the side of the upper electrode increase, the increasing coercive electric field can be suppressed.
Nevertheless, every one of the above techniques have a problem of an increase in leakage current because the ferroelectric film has a region where the Zr/Ti ratio is lower than that of the central section of that ferroelectric film in the direction of thickness.